Harvesting energy from sounds, human motions, stray electromagnetic signals, vibrating machines, etc. offers many advantages over other sources of energy such as solar cells that generate electrical energy from sunlight but require batteries when sunlight is not present. In the early 1900s, energy harvesters collected energy from vibration and were used as direct battery replacements in flash lights. This type of vibration energy harvester and others like it are based upon principles of transduction where a strong magnet is passed into and out of a tightly coiled wire thereby inducing a current in the coiled wire. In recent conventional devices, energy is stored in a capacitor which can be recharged several hundred thousand times to power the flash light. However, these type of conventional vibration energy harvesters have not been successful as a battery replacement in most electronic devices because of mechanical and electrical constraints. To the contrary, dependence on electronic devices powered by batteries has risen and at least over four billion people or half the world population have used batteries. This is a huge waste disposal issue and degrades the environment.
Vibration energy harvesting is promising but faces many difficulties. Challenges identified in vibration energy harvesting include: (1) a very narrow bandwidth which limits ability of a vibration energy harvester to be functional in divergent environments; (2) low power output; (3) the lack of a miniature light weight device with retained meaningful power output; and (4) how best to extract electrical power from the vibration energy harvester.
One example of a conventional vibration harvesting device is the Volture piezoelectric vibration energy harvester sold by Mide Technology Corporation, 200 Boston Ave, Suite 1000 Medford, Mass. 02155, United States of America (www.mide.com). The Volture piezoelectric energy harvester is a single cantilever beam where oscillation of the thin piezoelectric substrate causes strain thereby producing a voltage. To obtain maximum power which is in milliwatts, the Volture piezoelectric energy harvester must be tuned to match the resonant frequency of the vibrating source. Another example is the Perpetuum electromagnetic vibration energy harvester sold by Perpetuum Ltd, Epsilon House, Southampton Science Park, Southampton SO16 7NS, United Kingdom (www.Perpetuum.com). The Perpetuum electromagnetic vibration energy harvester utilizes the principles of electromagnetic induction to harvest vibrational energy at one resonant frequency to give maximum power which is also in milliwatts. Perpetuum electromagnetic vibration energy harvesters convert mechanical energy (vibration) to electrical energy via an oscillating mass (magnet) which traverses across a fixed coil creating a varying amount of magnetic flux, inducing a voltage according to Faraday's law. To maximize power output, the harvester is mechanically tuned to an optimized resonant frequency present within the application environment. To harvest at other frequencies, different device models of the Perpetuum electromagnetic vibration energy harvester are needed.
Almost all conventional vibration energy harvesters are based upon the design rules of William and Yates (C. B. Williams and R. B. Yates, “Analysis of a micro-electric generator for microsystems,” Sensors Actuators Phys., vol. 52, no. 1, pp. 8-11, 1996). The rules always lead to a narrow frequency response and make it difficult to tune the frequencies at which energies are harvested. Narrow bandwidth limits an energy harvesting device to one specific frequency only, leading to difficulty in operating at any other frequency and consequently non-deployable to diverse environments and diverse sources of energy. Based on the same rules, making a device small is at the expense of efficient and useful power generation. William and Yates rules are directly dependent on effective weight (mass) of movable parts of a harvester and strongly defines linear systems, extended later to equally define nonlinear systems (R. S. Langley, “A general mass law for broadband energy harvesting,” J. Sound Vib., vol 333, no. 3, pp 927-936, 2014). Microelectromechanical systems (MEMS) have been widely researched, which to-date are bound by the rules to a very low power output. On the other hand, macrosized vibration energy harvesters generate large amounts of power but many of them could be cumbersome and inconvenient as hand-held, wearable and pocketable devices. In addition, the intensity of the exciting source is required to be large enough in order excite a large sized vibration energy harvester.
The idea of introducing dynamic nonlinearities to vibration energy harvester designs was formed to address challenges facing the technology (A. Erturk, J. Hoffmann, and D. J. Inman, “A piezomagnetoelastic structure for broadband vibration energy harvesting,” Appl. Phys. Lett., vol. 94, no. 25, pp 254102, 2009; F. Cottone, H. Vocca, and L. Gammaitoni, “Nonlinear Energy Harvesting,” Phys. Rev. Lett., vol. 102, no. 8, pp 080601 [1-4], 2009; B. P. Mann and N. D. Sims, “Energy harvesting from the nonlinear oscillations of magnetic levitation,” J. Sound Vib., vol. 319, no. 1-2, pp 515-530, 2009). Common to all the intervening ideas is the requirement for high intensity of input excitation.
Added to these challenges is the conventional requirement of impedance matching for the extraction of power from a vibration energy harvester. What works best for each specific device is often overlooked, rendering harvested extracted power inefficient.